This invention relates to the fabrication of electronic devices, and more particularly to processes for forming strained Si and SiGe alloy films where the SiGe alloy has improved thermal conductivity.
Silicon layers that possess tensile strain are of interest for use in high-performance CMOS devices. Improved charge carrier mobilities in strained Si layers permit enhanced FET performance (higher on-state current) without the need for geometric scaling in the device. A strained Si layer is typically formed by growing a Si layer on a relaxed silicon-germanium (SiGe) layer. Depending on the device application, the SiGe layer may be grown either on a bulk Si substrate or on top of a silicon-on-insulator (SOI) wafer.
The strained Si on a relaxed SiGe layer may be viewed as a Si/SiGe bilayer structure.
Regardless of how the substrates are made, a substantial obstacle to device fabrication in a Si/SiGe bilayer structure is the poor thermal conductivity of the SiGe alloy material. This has been shown to degrade the DC characteristics of transistors fabricated on the bilayer structure. Since heat cannot be transported away as quickly as in the case of pure Si, the temperature in the channel region of the device increases, thereby degrading the mobility of the charge carriers.
In general, a variation in mass of the constituent atoms in a lattice reduces the phonon lifetime within the crystal, which in turn leads to reduced thermal conductivity. In the case of a SiGe random alloy, variation in mass between Si and Ge atoms, and among the various isotopes of Si and Ge, leads to reduced thermal conductivity. In a typical random SiGe alloy with naturally occurring Si and Ge, Si has three isotopes 28Si, 29Si and 30Si, and Ge has five isotopes 70Ge, 72Ge, 73Ge, 74Ge and 76Ge. The thermal conductivity of the SiGe material can be improved by using isotopically enriched gas sources for SiGe formation, which minimizes the isotopic mass variance of the Si and Ge respectively. U.S. Published patent application 2004/0004271 (Fukuda et al.) proposes that a SiGe layer be formed by deposition using silane (SiH4) and germane (GeH4) gases where the isotope concentration of 28Si and 70Ge are both greater than 95%. A layer of Si (which may also be isotopically enriched) is deposited over this SiGe layer. This technique results in a bilayer structure of strained Si on a relaxed SiGe alloy layer having reduced isotopic mass variance, on a bulk Si substrate or an SOI substrate. FIGS. 1 and 2 show application of this technique on an SOI substrate. A typical SOI substrate 10 has an insulator layer 2 and a Si layer 3 on a Si substrate 1 (FIG. 1). Source gases 21, 22 for isotopically enriched Si and Ge are used in a deposition process to form a random SiGe alloy layer 4 (FIG. 2). The isotopic enrichment serves to lower the mass variance of the SiGe layer, thereby improving its thermal conductivity.
A thermal mixing process may be employed to mix Si layer 3 with reduced-mass-variance SiGe layer 4, to produce a relaxed SiGe layer 5 on insulator 2 (FIG. 3). This structure may thus be viewed as a relaxed SiGe-on-insulator (SGOI) substrate, on which a Si layer 6 may be formed to provide a strained Si layer, as shown in FIG. 4.
In order to realize the advantages of strained Si layers in CMOS devices, there is a need to provide Si/SiGe bilayer structures with improved thermal conductivity in the SiGe alloy layer. It is desirable to form relaxed SiGe layers with reduced mass variance, without the added complexity and expense of using isotopically enriched source gases for the Si and Ge.